British passengers queue to leave the airport of the Red Sea resort of Sharm el-Sheikh, November 6, 2015.Asmaa Waguih

MOSCOW/WASHINGTON (Reuters) - Moscow suspended passenger flights to Egypt and the United States imposed new air travel security requirements in the wake of the crash of a Russian jet in Egypt, as Western officials pointed on Friday to the conclusion it was brought down by a bomb.

A group affiliated with Islamic State has claimed responsibility for the crash of an Airbus A321 operated by a Russian carrier on Saturday that was bringing holidaymakers home from a resort on Egypt's Sinai Peninsula.

All 224 people on board were killed in what the militants described as revenge for Russian air strikes in Syria that began more than a month ago.

While no official investigation has confirmed that claim of responsibility, countries have been cancelling flights and announcing new precautions, leaving tens of thousands of European and Russian tourists stranded at Red Sea resorts.

The U.S. Department of Homeland Security announced new security measures on Friday, including tighter screening of items before they are brought on board aircraft, for flights to the United States from some foreign airports in the region.

U.S. President Barack Obama and British Prime Minister David Cameron have already said the crash might have been caused by a bomb. Moscow initially rebuked Western countries for drawing such conclusions too quickly. But President Vladimir Putin's decision to suspend Russian flights suggests the Kremlin is no longer trying to avert attention from that theory.

The sound of an explosion could be heard on the black boxes recovered from the plane, according to an investigator who had access to them, French TV station France 2 said on its website. The investigator ruled out engine failure, it added.

British and U.S. spies intercepted "chatter" from suspected militants as well as internal communication about the incident from one other government that suggested a bomb, possibly hidden in luggage in the hold, had downed the airliner, Western intelligence sources said.

The intelligence sources, who spoke on customary condition of anonymity, said the evidence was not categorical and there was still no hard forensic or scientific evidence to support the bomb theory.

"We still cannot be categorical, but there is a distinct and credible possibility that there was a bomb," one source said.

A Sinai-based group affiliated with Islamic State, themilitant group that has seized swathes of Iraq and Syria, hasclaimed responsibility for the crash, which if confirmed wouldmake it the first attack on civil aviation by the world's mostviolent jihadist organisation.

But Moscow, which launched air strikes against Islamistfighters including Islamic State in Syria more than a month ago,has said it is premature to reach conclusions that the flight was attacked.

Egypt, which depends on tourism as a crucial source ofrevenue, says there was no evidence a bomb was to blame.

U.S. MEASURES

White House spokesman Josh Earnest said the new U.S. measures are a "prudent response" to boost security procedures already in place and will affect fewer than 10 airports.

All the airports being asked to tighten screening of U.S.-bound flights are in the Middle East, a U.S. official familiar with the matter said.

One U.S. official said that within the last two years, the U.S. Transportation Security Administration had assessed security at foreign airports, and that Sharm el Sheikh had a reputation inside the U.S. government for poor security.

Decisions by Britain and other European countries to suspend flights to Sharm al-Sheikh left tens of thousands of tourists stranded in one of the most popular destinations for European holidaymakers seeking winter sunshine. Moscow's decision to follow suit on Friday adds tens of thousands more.

Putin accepted advice from Russia's FSB security service to suspend passenger flights from Egypt while the cause of the crash was unclear, Kremlin spokesman Dmitry Peskov was quoted as saying by the Interfax news agency.

The Russian president agreed in a phone conversation with Egyptian counterpart Abdel Fattah al-Sisi to keep cooperating on flight safety, the Kremlin said.

He said the government would find a way to bring Russians back home and would open talks with Egyptian authorities to improve flight safety. Peskov later told reporters the suspension would remain in place until such time as the Kremlin was satisfied that security had been sufficiently improved.

"I think that since Putin made the decision to cancel flights, most likely there is a genuine suspicion that it was a terrorist act. And of course, then it is correct to cancel the flights because it means it is dangerous to fly there," said Maria Solomatina, 27, an IT consultant who has a ticket to travel to Egypt in mid-November.

The Russian Travel Industry Union, cited by Interfax, estimated there were about 50,000 Russian tourists currently in Egypt and said refunding cancelled tickets to Egypt could bankrupt Russian tour operators.

Tourist agency Tez Tour, which estimates it sells about 15 percent of trips to Egypt from Russia, said 10,000 of its Russian clients were in Egypt.

British attempts to bring home thousands of stranded tourists were thrown into chaos on Friday when Egypt reduced the number of flights it would allow to take them home.

Tour companies were trying to mount an operation to bring British tourists home with only hand baggage and fly their luggage separately, but Cairo restricted the number of flights.

Additional reporting by Jack Stubbs and Maria Tsvetkova in Moscow; Abdelnasser Aboelfadl in Cairo, Guy Faulconbridge in London and Mark Hoseball in Washington; Writing by Anna Willard and Alistair Bell; Editing by Peter Graff and Ken Wills